ISTANBUL (Reuters) - Turkish authorities have detained a brother of Fethullah Gulen, a U.S.-based cleric they accuse of orchestrating a coup attempt in July, and issued arrest warrants for 115 other people as part of nationwide investigations into the abortive putsch.

Kudbettin Gulen was taken into custody on Sunday by Turkish counter-terrorism police in the Gaziemir district of the Aegean province of Izmir following intelligence that he was staying at a relative's house, state-run Anadolu Agency said.

The daily Hurriyet reported that Kudbettin Gulen had been abroad recently but that security services learned he had returned to Turkey and he was then detained.

Several of Gulen's relatives, including a nephew, niece and cousins, have been arrested since the July 15 coup.

A chief prosecutor's office in Sakarya province, 150 km (93 miles) east of Istanbul, has opened an investigation into 148 people over their suspected links to Gulen's movement, Anadolu said. A court issued arrest warrants for 115 of them.

Police carried out simultaneous raids across 27 provinces to detain the suspects, it added.

Turkey has dismissed or suspended more than 100,000 people in the military, civil service, police and judiciary over their alleged role in the attempted coup. Some 32,000 people, including soldiers and journalists, have been arrested.

Ankara wants the United States to detain and extradite Gulen so that he can be prosecuted in Turkey on a charge that he masterminded the attempt to overthrow the government. Gulen, who has lived in self-imposed exile in Pennsylvania since 1999, denies any involvement.

The security crackdown has alarmed rights groups and Western allies who fear President Tayyip Erdogan may be using the failed coup as a pretext to curtail all dissent and intensify action against suspected sympathizer of Kurdish militants.